U.S. patent application number 12/932372 was filed with the patent office on 2012-08-30 for metal deposition using seed layers.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Yu Bo, Gang Chen, Shuo Chen, Hsien-Ping Feng, Bed Poudel, Zhifeng Ren.
Application Number | 20120217165 12/932372 |
Document ID | / |
Family ID | 46718264 |
Filed Date | 2012-08-30 |
United States Patent
Application |
20120217165 |
Kind Code |
A1 |
Feng; Hsien-Ping ; et
al. |
August 30, 2012 |
Metal deposition using seed layers
Abstract
Methods of forming a conductive metal layers on substrates are
disclosed which employ a seed layer to enhance bonding, especially
to smooth, low-roughness or hydrophobic substrates. In one aspect
of the invention, the seed layer can be formed by applying
nanoparticles onto a surface of the substrate; and the
metallization is achieved by electroplating an electrically
conducting metal onto the seed layer, whereby the nanoparticles
serve as nucleation sites for metal deposition. In another
approach, the seed layer can be formed by a self-assembling linker
material, such as a sulfur-containing silane material.
Inventors: |
Feng; Hsien-Ping;
(Watertown, WA) ; Chen; Gang; (Carlisle, MA)
; Bo; Yu; (Chestnut Hill, MA) ; Ren; Zhifeng;
(Newton, MA) ; Chen; Shuo; (Newton, MA) ;
Poudel; Bed; (Newtonville, MA) |
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
GMZ Energy, Inc.
Waltham
MA
The Trustees of Boston College
Chestnut Hill
MA
|
Family ID: |
46718264 |
Appl. No.: |
12/932372 |
Filed: |
February 24, 2011 |
Current U.S.
Class: |
205/135 ;
205/118; 205/162; 205/183; 205/184 |
Current CPC
Class: |
C25D 5/022 20130101;
C25D 5/10 20130101; C25D 5/54 20130101 |
Class at
Publication: |
205/135 ;
205/183; 205/184; 205/162; 205/118 |
International
Class: |
C25D 5/10 20060101
C25D005/10; C25D 5/02 20060101 C25D005/02; C25D 5/54 20060101
C25D005/54 |
Goverment Interests
STATEMENT REGARDING U.S. GOVERNMENT RIGHTS
[0001] This invention was made with U.S. government support under
Grant No. DE-FG02-08ER46516, awarded by the U.S. Department of
Energy (DOE) and Grant No. CMMI-0833084, awarded by the National
Science Foundation (NSF). The U.S. Government has certain rights in
the invention.
Claims
1. A method of forming a conductive metal contact on a substrate,
comprising: applying a seed layer of nanoparticles onto a surface
of the substrate; and electroplating an electrically conducting
metal onto the seed layer, whereby the nanoparticles serve as
nucleation sites for metal deposition.
2. The method of claim 1, wherein the method further comprises:
applying the nanoparticles as a complex with an immobilizing
carrier; and annealing the complex to apply the nanoparticles onto
the surface of the substrate.
3. The method of claim 2, wherein the annealing step further
comprises sintering the nanoparticles onto the surface of the
substrate.
4. The method of claim 2, wherein the depositing step further
comprises applying a complex of polymer encased nanoparticles.
5. The method of claim 4, wherein the complex comprises at least
one polymer selected from the group of poly(vinylpyrrolidone)
(PVP), poly(acrylamide) (PAM), poly(vinyl alcohol) (PVAL),
poly(acrylic acid) (PAA), and poly(ethyleneimine) (PEI).
6. The method of claim 4, wherein the nanoparticles comprise metal
nanoparticles.
7. The method of claim 6, wherein the metal nanoparticles comprise
at least one metal selected from the group of platinum, gold,
palladium, ruthenium, silver, or nickel.
8. The method of claim 1, wherein the method further comprises
contacting the substrate with a surfactant prior to depositing the
seed layer.
9. The method of claim 8, wherein the surfactant further comprises
at least one cationic surfactant.
10. The method of claim 1, wherein the substrate is characterized
by at least one of low surface energy, poor wettability, a
hydrophobic surface, a glass material, low surface roughness.
11. The method of claim 1, wherein the method further comprises
selectively applying a seed layer on a portion of the surface and
preferentially depositing the electrically conducting metal on the
selectively applied seed layer.
12. The method of claim 11, wherein the step of selectively
applying a seed layer on the portion of the surface employs at
least one of photolithography, screen printing, inkjet printing,
micro-contact stamping and dip-pen nanolithography.
13. The method of claim 12, wherein the step of preferentially
depositing the electrically conducting metal further comprises
selecting a voltage at which electrically conduction material is
preferentially deposited on the selectively applied seed layer.
14. A method of forming a conductive metal contact on a substrate,
comprising: applying a seed layer of a linker material onto a
surface of the substrate; and electroplating an electrically
conducting metal onto the seed layer, whereby the seed layer
enhances grain homogeneity during metal deposition.
15. The method of claim 14 wherein the step of depositing a seed
layer further comprises applying an electron-donating linker
material.
16. The method of claim 14 wherein the step of depositing a seed
layer further comprises applying a sulfur containing linker
material.
17. The method of claim 14 wherein the step of depositing a seed
layer further comprises applying a self-assembling monolayer of
linker material.
18. The method of claim 14, wherein the step of applying the seed
layer comprises contacting a surface of the substrate with a
material comprising at least one sulfur-containing linker selected
from (3-mercaptopropyl-trimethoxysilane)
(3-Mercaptopropyl)methyldimethoxysilane,
(3-Mercaptopropyl)triethoxysilane, 3-Mercaptopropyl-functionalized
silica gels, (3-Chloropropyl)trimethoxysilane,
(3-Bromopropyl)trimethoxysilane, and
(3-Iodopropyl)trimethoxysilane.
19. The method of claim 14, wherein the step of applying the seed
layer comprises contacting a surface of the substrate with a
material comprising (3-mercaptopropyl)-trimethoxysilane (MPS).
20. The method of claim 14, wherein the step of electroplating a
metal onto the seed layer further comprises electroplating at least
one metal selected from the group of titanium, tantalum, tungsten,
aluminum, chromium, nickel, cobalt, silver, gold, copper, and their
alloys.
21. The method of claim 11, wherein the step of selectively
applying a seed layer on the portion of the substrate surface
further comprises: applying polymeric particles in an array pattern
on the substrate to cover a portion of the surface; depositing a
mask layer onto the polymeric particles and substrate surface;
removing the layer of polymeric particles thereby exposing the
portion of the substrate according to the array pattern; applying a
seed layer of nanoparticles to the remaining portions of the mask
layer and the exposed regions of the substrate surface; removing
the mask layer thereby removing a portion of the seed layer
adherent to the mask layer but preserving another portion of the
seed layer applied to the substrate according to the array
pattern.
22. The method of claim 21, wherein the polymeric particles are
self-assembled polystyrene microspheres.
23. The method of claim 21, wherein the mask layer is a layer of
copper.
24. The method of claim 21, wherein the step of removing the mask
layer comprises dissolving the mask layer with an acidic
solution.
25. The method of claim 21, wherein the step of applying a seed
layer of nanoparticles further comprises applying the nanoparticles
as a complex with an immobilizing carrier and annealing the complex
to apply the nanoparticles onto the surface of the substrate.
26. The method of claim 21, wherein the step of applying polymeric
particles further comprises etching the self-assembled polymer
clusters to a predetermined diameter.
Description
BACKGROUND OF THE INVENTION
[0002] Metallization is the process of depositing metal material on
the surface of a substrate. Electroplating and other forms of
electro-deposition are commonly used metallization techniques to
form electrical conductive contacts or protective coatings. For
example, electroplating is used in ultra large-scale integration
(ULSI) to provide multiple levels of copper or copper alloy
metallization.
[0003] Metallization is also used in the fabrication of
optoelectronic devices such as transparent thin film transistors
(TFT), flat panel displays, light-emitting diodes (LED),
photovoltaic cells, and electrochromic windows to provide
interconnects for transparent device-to-device integration. In the
case of electrochromic window fabrication, substrates are typically
semiconductors or transparent conductive oxides (TCO), e.g. zinc
oxide, indium-tin oxide, and fluorine-doped tin oxide. Furthermore,
electroplating is used in the fabrication of power electronics for
the metallization of ceramic substrates.
[0004] The fabrication of thermoelectric devices or device
components also requires metallization on semiconductor substrates.
Thermoelectric devices are uniquely advantageous in heat removal
and energy harvesting applications because they are free of moving
parts, acoustically silent, and they can be integrated into
microelectronic devices. Recent advances have greatly improved the
thermoelectric figure-of-merit (ZT) of nanostructured
thermoelectric alloys. In particular, nanostructured p-type bismuth
antimony telluride has achieved a peak figure-of-merit (ZT) of
about 1.4 at 100.degree. C.
[0005] However, these material property advances have not fully
translated into better overall performance in the thermoelectric
devices due at least in part to variations in the thermal and
electrical contact resistances between the nanostructured alloy
substrates and the metallized electrodes. A poorly formed contact
generates localized Joule heating effects and leads to a
non-uniform current distribution which lowers an effective
figure-of-merit (ZT.sub.eff) for the thermoelectric device from
that of the thermoelectric material.
[0006] Generally, in the fabrication of electronic devices,
thermoelectric devices, and other metallization processes,
desirable electrical and thermal contact properties are highly
correlated with a uniform deposition of the metallic atoms on the
substrate which creates a strong adhesion and an effective
interface between the deposited metal layers and the substrate. In
particular, the process of electroplating metal depends on a
nucleation process, which is determined by the formation energy,
excess energy, and internal strain energy of the phase transition
during metallization.
[0007] Direct electroplating on smooth, low-roughness, or
hydrophobic surfaces of glass, semiconductor, or ceramic substrates
is difficult because the target surface has low surface energy or
poor wettability, which leads to a relatively high excess energy
for electroplating nucleation. As a consequence, scattered and
irregular grains of metal grow on a small number of nucleation
sites, causing poor interfacial adhesion and large surface
roughness. A further consequence of the scattered and irregular
grain formation is that strain energy, which is caused by a
different atomic arrangement between two adjacent metallization
layers, increases with increasing overall metallization thickness,
and can sometimes cause metallization layers to spontaneously peel
off.
[0008] Furthermore, for many applications, metallization is desired
on only a portion of the substrate surface, e.g., to form an
electrical contact at a specific location. Here, additional
processes are typically employed prior to the electroplating
process to achieve a selective metallization. For example, a
patterning process can be used to form a masking pattern layer on a
selected region or regions on the surface of the substrate.
[0009] In one commonly used approach to selective metallization,
photolithography is employed to create a patterned photoresist
layer on the substrate. The exposed regions (the portions not
covered by the photoresist mask) can then be etched to create
additional surface roughness (or simply to expose the area for
metal deposition). Metal can then be sputtered onto the exposed
(and etched) regions. These processes improve both the adhesion and
electrical conductivity of the primary structure constructed by the
subsequent electroplating process. Following the electroplating
process, an additional chemical mechanical polishing process can be
used to remove any surplus metal and planarize the entire surface.
Finally, the photoresist is removed.
[0010] Although photolithography and other similar techniques can
achieve selective metallization with a high degree of precision,
these techniques require costly specialized equipments and can
significantly hinder device production rates.
[0011] There exists a need for better metallization techniques,
especially techniques that can be used on smooth, low-roughness or
hydrophobic substrates to achieve high quality metal layers with
strong adhesion. Metallization techniques that can achieve
selective metallization without the complexity of photolithography
would also satisfy a long felt need in the art.
SUMMARY OF THE INVENTION
[0012] Methods of forming a conductive metal layers on substrates
are disclosed which employ a seed layer to enhance bonding,
especially to smooth, low-roughness or hydrophobic substrates. In
one aspect of the invention, the seed layer can be formed by
applying nanoparticles onto a surface of the substrate; and the
metallization can be achieved by electroplating an electrically
conducting metal onto the seed layer, whereby the nanoparticles
serve as nucleation sites for metal deposition. In another
approach, the seed layer can be formed by a self-assembling linker
material, such as a sulfur-containing silane material.
[0013] The methods of the present invention can be particularly
effective to provide metallization to substrate surfaces that are
characterized by at least one of the following characteristics: low
surface energy, poor wettability, a hydrophobic surface, a glass
(or glass-like) composition or low surface roughness.
[0014] When nanoparticles are used as the linker material, the
nanoparticles can be applied as a complex with an immobilizing
carrier. Following application the complex can be annealed or
heated to essentially sinter the nanoparticles onto the surface of
the substrate. In one embodiment, the complex can be a complex of
polymer encased nanoparticles in which the polymer can be at least
one polymer selected from poly(vinylpyrrolidone) (PVP),
poly(acrylamide) (PAM), poly(vinyl alcohol) (PVAL), poly(acrylic
acid) (PAA), and poly(ethyleneimine) (PEI).
[0015] In certain preferred embodiments, the nanoparticles can be
metal nanoparticles, e.g., having a composition that includes at
least one metal selected from the group of platinum, gold,
palladium, ruthenium, silver, titanium, tantalum, tungsten,
aluminum, chromium, cobalt, nickel, and their respective alloys. In
some applications the method can further include a pre-treatment
step of contacting the substrate with a surfactant, e.g., a
cationic surfactant, prior to depositing the seed layer.
[0016] The methods of the present invention, especially when
metallic nanoparticles are used, can also be effective in selective
metallization of portion of a substrate by selectively applying a
seed layer to a portion of the surface and then preferentially
depositing the electrically conducting metal on the selectively
applied seed layer. For example, the seed layer can be selectively
applied to a portion of the surface by employing photolithography,
screen printing, inkjet printing, micro-contact stamping or dip-pen
nanolithography or combinations of such techniques. In certain
preferred embodiments, the seed layer can be selectively applied to
a portion of the surface by first applying a mask layer on a
substrate surface thereby exposing only a predetermined portion of
the substrate surface for applying the seed layer. The step of
preferentially depositing the electrically conducting metal can be
enhanced by selecting a voltage at which electrically conduction
material can be preferentially (or only) deposited on the
selectively applied seed layer.
[0017] The present invention also encompasses methods of a
chemically modifying the substrate with a linker material, such as
a sulfur or halogen containing silane. In some embodiments, such
chemical linkers can be applied to self-assemble into a monolayer
of linker material. For example the linker material can be a
sulfur-containing silane material such as
(3-mercaptopropyl-trimethoxysilane)
(3-Mercaptopropyl)methyldimethoxysilane,
(3-Mercaptopropyl)triethoxysilane, 3-Mercaptopropyl-functionalized
silica gel, (3-Chloropropyl)trimethoxysilane,
(3-Bromopropyl)trimethoxysilane, or (3-Iodopropyl)trimethoxysilane.
In one preferred embodiment, the sulfur-containing silane material
includes (3-mercaptopropyl-trimethoxysilane) (MPS).
[0018] The methods of the present invention can be used to provide
metallization with a wide variety of metals, including for example,
one or more of the following metals: titanium, tantalum, tungsten,
aluminum, chromium, nickel, cobalt, silver, gold, copper, and their
alloys.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIGS. 1A-1C illustrate a method of metal deposition using a
seed layer according to the invention;
[0020] FIGS. 2A-2C are atomic force microscopy (AFM) images
demonstrating metal deposition according to the invention;
[0021] FIGS. 3A-3B depict a cathodic waveform and a series of
photographs that demonstrate selective metal depositions according
to the invention;
[0022] FIGS. 4A-4C depict another method of selective metal
deposition according to the invention;
[0023] FIGS. 5A-5C depict a series of cathodic waveform that
demonstrate the selective deposition of various metals according to
the invention;
[0024] FIGS. 5D-5E depict chronoamperograms of the selective
deposition of various metals according to the invention;
[0025] FIG. 5F depicts a cathodic waveform of the annealing process
according to the invention;
[0026] FIGS. 6A-6G depict another method of selective metal
deposition according to the invention;
[0027] FIGS. 7A-7F are scanning electron microscope (SEM) and AFM
images confirming a selective metal deposition according to the
second embodiment;
[0028] FIGS. 8A-8C illustrate a method of metal deposition on
semiconductor substrates for fabricating thermoelectric devices,
according to the invention;
[0029] FIGS. 9A-9B depict a cathodic waveform and a
chronoamperogram during a metal deposition performed according to
the method shown in FIGS. 8A-8C;
[0030] FIGS. 10A-D are SEM and AFM images confirming the metal
deposition performed according to the method shown in FIGS.
8A-8C;
[0031] FIGS. 11A-11F depict SEM images of metal deposition with
various concentrations of nanoparticle seed layers, according to
the invention.
DETAILED DESCRIPTION
[0032] The following definitions provide additional context for the
detailed descriptions that follow and are not intended to limit the
scope of the detailed descriptions.
[0033] The term "substrate" as used herein is intended to encompass
electronic materials, such as semiconductor and thermoelectric
materials, as well as inert materials, such as glasses, ceramics
and dielectrics.
[0034] The term "surface" as used herein is intended to encompass
an entire surface of a substrate or a selected region of the
substrate. For example, the seed layers described herein may be
selectively deposited onto regions of the substrate to form
discontinuous surfaces for selective metallization.
[0035] The term "metal" as used herein is intended to encompass
elemental metals and metal alloys, as well as metallic compounds
and metal precursors that can be used to form a conductive contacts
and/or nanoparticles.
[0036] The term "self-assembling polymer" as used herein is
intended to encompass any polymer that arranges in an ordered state
as a solution of the polymer approaches equilibrium, thereby
reducing its free energy. Examples include:
3-mercaptopropyl-trimethoxysilane (MPS),
(3-Mercaptopropyl)methyldimethoxysilane,
(3-Mercaptopropyl)triethoxysilane, 3-Mercaptopropyl-functionalized
silica gel, (3-Chloropropyl)trimethoxysilane,
(3-Bromopropyl)trimethoxysilane,
(3-Iodopropyl)trimethoxysilane.
[0037] The term "grain homogeneity" as used herein is used to
describe a measure of microstructural uniformity of regularity in
grain boundary, grain size, or crystalline geometry
[0038] The term "linker material" as used herein is used to
describe molecules, monomer, or functional groups on molecules or
monomers that provide chemical bond or electrical affinity between
neighboring molecules or monomers.
[0039] The term "bifunctional linker" as used herein is used to
describe any molecule or monomer that has two functional groups, or
binding sites, or has affinity for two groups of atoms or
molecules.
[0040] The term "electrically conducting linker material" as used
herein is used to describe a molecule or monomer that conducts
electron into a reaction center thereby stabilizes an electron
deficient molecule or monomer, for example, a cation.
[0041] The term "self-assembled monolayer" as used herein is used
to describe an organized layer of amphiphilic molecules in which
one end of the molecule, the "head group," provides a special
affinity for a substrate, and in which a second end of the
molecule, the "tail group," provides a functional group as a
terminal end. The SAM is created by the chemisorption "head groups"
onto a substrate from either the vapor or liquid phase followed by
a substantially planar organization of "tail groups".
[0042] In the fabrication of semiconductor and/or thermoelectric
devices, conventional metallization techniques such as sputtering
and electroplating are problematic in a number of aspects. In the
case of sputtering, conventional methods require costly vacuum
equipments and results in a low production rate. For example, the
most widely used thermoelectric materials near room temperature,
bismuth telluride and its alloys (Bi.sub.2Se.sub.yTe.sub.3y as the
n-type, Bi.sub.2Sb.sub.2xTe.sub.3 as the p-type), form complex
semiconductor-to-metal interfaces. These Bi.sub.2Te.sub.3-based
alloys are small bandgap thermoelectric semiconductors with a low
melting point (573C), a low surface energy (hydrophobic), and a low
resistivity (100-1000 .mu..OMEGA.-cm). In theory, the small bandgap
property (0.11-0.16 eV) provides an ohmic contact with a low
specific contact resistance (10e-7 Ohm cm.sup.2), which are ideal
properties for the fabrication of a semiconductor-to-metal contact.
However, the hydrophobic property of the substrate surface requires
an additional thin layer to improve adhesion. For example, chromium
is typically applied by sputtered before a nickel contact is
deposited onto Bi.sub.2Te.sub.3-based alloys. The surface of
nanostructured Bi.sub.2Te.sub.3-based alloys exhibit several times
more grain boundaries and a disordered orientation than bulk
material, thereby presenting a broad and chaotic distribution of
surface energy that is unfavorable to subsequent metallization.
[0043] As a result of the above problems with sputtering
techniques, electrochemical deposition is widely use instead.
However, all of commonly used electrochemical deposition
techniques, electroplating techniques, and electroless plating
techniques results in poor metallization on semiconductor
substrates.
[0044] In the case of electroplating techniques, thermoelectric
semiconductors, such as Bi.sub.2Te.sub.3-based alloys have a lower
surface energy than metals leading to a poor wettability and a
higher nucleation formation energy. The resulting scattered growth
of nuclei on a small number of nucleation sites cause weak
interface contact and poor adhesion. Nanostructured surfaces can
have more nucleation sites for electroplating, however, the broad
distribution of surface energy can cause electroplating nucleation
to be more uneven.
[0045] In the case of electroless plating techniques, nickel is
plated on Bi.sub.2Te.sub.3-based substrates by using an initial
coating of palladium-tin catalyst. However, this method results in
a relatively high contact resistivity because unwanted impurities
(the palladium-tin used as a plating catalyst, the reducing agent,
and the chelating agent) and structural imperfections accumulate at
the interface. For Bi.sub.2Te.sub.3-based alloys, the acidic
solution of the palladium-tin catalyst also oxidizes and corrodes
the substrate surface according to the Pourbaix diagram. These
nanostructured surfaces are particularly susceptible to damage at
their weak points in the grain boundaries.
[0046] A third approach, direct electroplating, is a hybrid method
that utilizes aspects of both electroplating and electroless
plating techniques to deposit metal on a nonconductor or
semiconductor. Direct electroplating starts by coating an activated
palladium and further enhances the activated palladium colloid by
dipping the colloid in a sulfide-containing solution. The presence
of sulfide forms a link between the metal ions and the palladium,
thereby allowing electrical current to pass during electroplating.
Other compounds with the electron bridging property similar to
sulfide can be used as such electroplating promotion agent.
Unfortunately, the palladium-tin colloids used in these direct
electroplating methods remain as sources of structural weakness, as
discussed in the case of electroless plating.
[0047] Accordingly, better techniques for metallization are
desired. In one aspect of the present invention, nanoparticles are
used to provide a seed layer for metal deposition. As shown in
FIGS. 1A-1C, a substrate 100 for metal deposition can be a
fluorine-doped tin oxide (FTO) glass. A cut piece of FTO of 1
cm.sup.2 in area and 2 mm in thickness have the physical
characteristics of a sheet resistance of 10 .OMEGA./square, a
resistivity of 350 .mu..OMEGA.cm, a band gap potential of 3.8 eV,
and an optical transmissivity of 90%.
[0048] First, as shown in FIG. 1A, a deposition surface 101 of the
FTO glass substrate 100 can be cleaned. According to one preferred
embodiment, the FTO glass substrate 100 can be immersed in a
solution of 2% PK-LCG545 (Parker Corp.) at 50.degree. C. for 5
minutes with sonication to clean the deposition surface 101.
[0049] Next, as shown in FIG. 1B, the deposition surface 101 can be
pretreated by depositing a seed layer of nanoparticles 102.
According to a preferred embodiment, a layer of a nanoparticles
complex in an immobilizing carrier can be deposited onto the
deposition surface 101 by immersing the substrate in a solution of
surfactants followed by immersing the substrate in a solution of
immobilized nanoparticles complexes. Further according to the
preferred embodiment, the substrate 100 can be immersed in a 1%
surface conditioner (2-(2-Aminoethylamino) ethanol, AEEA) at
45.degree. C. for 5 minutes, and then immersed in a solution of
PVP-capped platinum nanoclusters suspension at room temperature for
5 minutes.
[0050] Here, platinum nanoparticles can be used because platinum
can be an inert metal and its suspension can be readily prepared
without additional purification. Although platinum nanoparticles
are used in the preferred embodiment, other nanoparticle materials
can be used to provide a seed layer of nanoparticles.
[0051] The PVP-capped platinum nanoclusters can be synthesized by
the following procedure: forming a PVP solution by dissolving 0.1
grams Poly-N-vinyl-2-pyrrolidone (PVP) (MW=8000) in 44 ml deionized
water at room temperature in a beaker with stirring; adding a
precursor of H.sub.2PtCl.sub.6 (0.2 grams) to the PVP solution,
thereby obtaining a ratio of weight of polymer (PW) to a weight of
noble metal (MW) of about 1.1; finally, adding a 5 ml reductant
(0.5M NaBH.sub.4 solution) slowly to the solution. The solution
quickly changes from yellowish to black, indicating the formation
of Pt nanoclusters. The whole procedure can be performed at room
temperature within 30 minutes.
[0052] Next, the deposition surface 101 and the seed layer 102 of
nanoparticles thereon are annealed to sinter the seed layer of
nanoparticles, thereby form a pretreated deposition surface.
[0053] According to a preferred embodiment, the pretreated
deposition surface 101 can be annealed at 250.degree. C. for 10
minutes at ambient conditions to sinter the platinum nanoparticles
and to burn away the protective PVP-polymers.
[0054] Next, as shown in FIG. 1C, a metal material can be
electroplated onto the pretreated deposition surface 101 to form a
metallization layer 103. According to a preferred embodiment,
Nickel can be electroplated onto the pretreated deposition surface
101 within an optimum range of the operating voltage from a
commercial electrolyte at 53.degree. C. Other metals can be
electroplated onto the pretreated deposition surface, for example,
copper, tin, and gold can be similarly electroplated at room
temperature.
[0055] FIGS. 2A-2C are images of atomic force microscopy (AFM)
which confirm the metal deposition as described above. In
particular, FIG. 2A is an AFM image of an untreated FTO substrate
surface. FIG. 2B is an AFM image of an approximately 100 nm layer
of nickel electroplated on an untreated FTO substrate surface.
Finally, FIG. 2C is an AFM image of an approximately 100 nm layer
of nickel electroplated on a FTO substrate pretreated with a seed
layer of platinum nanoparticles. The AFM images are produced with a
scan size of 10 .mu.m and a scan rate of 1 Hz. The scale on the
z-axis is 1 .mu.m/div and the scale on the x-axis is 2
.mu.m/div.
[0056] A root-mean-square (RMS) thickness measurement can be made
in each of the AFM images in FIGS. 2A-2C. Specifically, FIG. 2A
depicts an untreated substrate having an RMS thickness of 25.34 nm.
FIG. 2B depicts an untreated substrate having an RMS thickness of
58.11 nm after deposition. Finally, FIG. 2C depicts a pretreated
substrate having an RMS thickness of 29.56 nm after deposition. In
particular, FIGS. 2A and 2C show that the RMS value of roughness of
the electroplated substrate can be made substantially the same as
the untreated substrate. Therefore, the above-discussed method of
providing a seed layer of immobilized nanoparticles on a substrate
provides a well-adhered nucleated layer to produce a reliable and
uniform electroplated metal film.
[0057] According to another aspect of the preferred embodiment, a
metal can be selectively deposited on a predetermined region or
regions of a substrate. The enabling electrochemical
characteristics according to this aspect of the preferred
embodiment is discussed below with respect to FIGS. 3-4.
[0058] FIG. 3A depicts two cathodic waveforms that result from
performing the analytical technique of cyclic voltammetry for
electroplating tin on a FTO substrate. The analytical technique of
cyclic voltammetry is typically used to confirm the electrochemical
properties of the metal electrolyte in the electroplating solution.
Here, a Gamry PCI4/300 potentistat/galvanostat can be used in the
electrochemical measurements in a standard three-electrode system
having a platinum mesh as the counter electrode and a saturated
calomel electrode (SCE) as the reference electrode.
[0059] As shown in FIG. 3A, waveform A illustrates the
electrochemical measurements of electroplating tin with a seed
layer of platinum nanoparticles, and waveform B illustrates the
electrochemical measurements of electroplating tin without the seed
layer. As shown, the current-potential curve of tin on pretreated
FTO glass (waveform A) is less negative than the current-potential
curve on a untreated surface (waveform B).
[0060] The seed layer of nanoparticles deposited on FTO serve as
nucleation sites to raise the surface energy such that when the
metal atoms are deposited onto the nanoparticles, the total surface
energy can be reduced. Therefore, the seed layer of deposited
nanoparticles cause the current-potential curve on a pretreated
substrate to be less negative than an untreated substrate. This
allows the metal atoms to more strongly adhere to the substrate
rather than to each other.
[0061] As shown in FIG. 3A, a region defined by a dash box C
illustrates an operational window for selective deposition in a
predetermined region or regions of a substrate that can be
pretreated with a seed layer of nanoparticles. For example, as
shown in FIG. 3A, electroplating tin on an untreated FTO surfaces
requires a larger onset potential of E.sub.onset=-0.55 V to
initiate deposition when compared with electroplating the same on a
FTO surface pretreated with nanoparticles (E.sub.onset=-0.27 V). As
another example, as shown in FIG. 5A, electroplating nickel on a
untreated FTO surfaces requires a larger onset potential of
E.sub.onset=-1.0 V to initiate deposition when compared with
electroplating the same on a FTO surface pretreated with
nanoparticles (E.sub.onset=-0.53 V).
[0062] Accordingly, various electroplating voltage conditions are
shown in FIG. 3B. In particular, a circular region of a FTO
substrate can be deposited with a layer of nanoparticles, and the
substrate can be electroplated with tin at various voltage levels,
i.e. -0.2V, -0.4V, -0.53V, -0.65V, and -1.0V. As shown,
electroplating under voltages that fall within the voltages defined
by the dash box C in FIG. 3A exhibits selective metallization in
the circular region. In contrast, outside the operational window as
defined by dash box C in FIG. 3A, metallization either fails to
occur (-0.2V) or occurs without selectivity (-1.0V).
[0063] Therefore, a method of selective metal deposition is
provided according to another aspect of a preferred embodiment.
Specifically, if electroplating is performed within the voltage
range of approximately -0.27V and -0.53V, only a selected portion,
the circular region, of the glass substrate is metalized with
tin.
[0064] As shown in FIGS. 4A-4C, a metallization layer can be
selectively formed by an electroplating process. In particular,
predetermined regions 101a and 101b of a deposition surface 101 of
the substrate 100 are selected for electro-deposition by a
deposition of a seed layer. As shown in FIG. 4A, a number of
processes can be used to selectively deposit a surfactant film,
including photolithography, screen printing, inkjet technology,
microcontact stamp printing, dip-pen nanolithography, and
electrochemical imprinting.
[0065] Next, as shown in FIG. 4B, the deposition surface 101 can be
pretreated by depositing a seed layer of nanoparticles 102a and
102b in predetermined regions 101a and 101b, respectively.
[0066] Next, the pretreated deposition surface 101 and the seed
layer 102a and 102b of nanoparticles thereon are annealed to sinter
the seed layer of nanoparticles. According to a preferred
embodiment, the pretreated deposition surface 101 can be annealed
at 250.degree. C. for 10 minutes at ambient conditions to sinter
the platinum nanoparticles and to burn away the protective
PVP-polymers.
[0067] Next, as shown in FIG. 4C, a metal can be electroplated onto
the pretreated deposition surface 101 to form a metallization layer
103a and 103b. According to a preferred embodiment, Nickel can be
electroplated onto the pretreated deposition surface 101 within an
optimum range of the operating voltage from a commercial
electrolyte at 53.degree. C. Other metals can be electroplated onto
the pretreated deposition surface, for example, copper, tin, and
gold can be similarly electroplated onto to pretreated substrate
surface 101 at room temperature.
[0068] A similar trend can be found in electroplating nickel,
copper, and gold, shown in FIGS. 5A-5C, respectively. Nickel
electrodeposition onto a bulk platinum surface requires a less
negative onset potential (-0.75V) (FIG. 5A) compared to the blank
FTO surface (-1.0V), which indicates that atomic clusters bond more
strongly on platinum. Furthermore, nickel deposited on platinum
nanoparticles has less negative onset potential (-0.53 V) than that
on bulk platinum surface. This underpotential is because nano-sized
platinum particles have high surface energy, and thus serve as
nucleation sites for atomic clusters to preferentially deposit on
them to reduce the total surface energy. The deposited atoms will
be more strongly bound to the nanoparticles which were strongly
immobilized onto substrate, leading to stronger adhesion.
[0069] As discussed above, similar operational windows are provided
for other electroplating metals. Accordingly, FIGS. 5A-5C depict
waveforms for nickel, copper, gold, respectively. Specifically, in
each of FIGS. 5A-5C, waveform A illustrates the electrochemical
measurements of electroplating the respective metal with the seed
layer of platinum nanoparticles, and waveform B illustrates the
electrochemical measurements of electroplating the respective metal
without the seed layer. And the dash boxes illustrate operational
windows for selective metallization of the respective metals.
[0070] Furthermore, as shown in FIG. 5C, waveform D corresponds to
the electrochemical measurements during gold deposition onto nickel
which was originally electroplated on the nanoparticle regions. As
shown, it is also more positive when compared with electroplating
gold on an untreated FTO substrate. Thus a second operational
window for the subsequent selective electroplating of gold is
provided, and a double-layer structure can be achieved. It is
understood that other metal can be electroplate instead or in
addition to gold.
[0071] According to another aspect of preferred embodiment, optimum
temporal parameters are provided for the electroplating process,
and is discussed below with respect to FIGS. 5D and 5E. As shown in
FIG. 5D, a chronoamperograms can be obtained to confirm the current
response to the optimum operating voltage at -0.8 V for
electroplating nickel on pretreated FTO (waveform A) and untreated
FTO (waveform B). In particular, waveform A shows an average rate
of electroplating nickel on FTO with the Pt nanoparticle treatment
can be about 0.2 .mu.m/min (as measured by a profilometer). In
contrast, almost no faradic current can be detected for
electroplating on untreated FTO, which confirms that no substantial
deposition has occurred. Similarly, as shown in FIG. 5E, a
consistent results is seen in the chronoamperograms for
electroplated tin under operating voltage of -0.4 V.
[0072] According to another aspect of the preferred embodiment, a
wide range of optimum concentration and deposition time are
provided for desirable metal deposition. As shown in FIGS. 11A-11F,
SEM images are taken for of nickel electroplating nickel at various
time durations and with various concentrations of nanoparticle
solutions. Specifically, FIGS. 11A, 11B, 11C, and 11D depict
electroplating nickel for durations of 0 second, 5 seconds, 15
seconds, and 30 seconds on untreated FTO surfaces; FIGS. 11E, 11F,
11G, and 11H depict electroplating nickel for durations of 0
second, 5 seconds, 15 seconds, and 30 seconds on FTO surfaces
pretreated by platinum nanoparticles; and FIGS. 11I, 11J, 11K, and
11L depict electroplating nickel for durations of 0 second, 5
seconds, 15 seconds, and 30 seconds on FTO surface pretreated with
a solution of 20% diluted platinum nanoparticles. These SEM images
confirm that a substantially uniform application of nanoparticles,
which provide uniform nucleation sites, can be obtained with
solutions of a wide range of concentrations of nanoparticles.
[0073] According to another aspect of the preferred embodiment,
optimum temporal parameters are provided for the annealing step. As
shown in FIG. 5F, a cathodic waveform cyclic voltammetry is
obtained for nickel electroplated on FTO with platinum
nanoparticles treated by various annealing times. As shown,
electroplating on a nanoparticle-pretreated surface before
annealing occurs at the same onset potential of electrodeposition
on untreated FTO surface. The reason could be the poor contact
between nanoparticles and substrate caused by the existence of the
protective PVP-polymer and conditioner resulting in higher
charge-transfer resistance. If annealing is performed for over 10
minutes at 250.degree. C., the protective PVP-polymer and
conditioner can be burned away so that the current-potential curve
moved forward less negative and opened up a window of selective
electroplating.
[0074] It is also observed that nanoparticles on FTO without
annealing can be wiped off while no obvious nanoparticles being
wiped off after annealing. Puff-off test also showed that
insufficient annealing time would cause poor bonding between the
nanoparticles and the substrate leading to coating came off.
[0075] FIGS. 6A-6G depicts a second embodiment for selective
metallization. According to the second embodiment, a low-cost
technique based on self-assembled polystyrene microspheres can be
used to pattern a highly-ordered dot arrays on a substrate 600, in
order to selectively metallize a portion of the substrate surface.
As shown in FIG. 6A, a monolayer of polystyrene microspheres 610
can be selectively deposited in predetermined regions on a FTO
substrate 600. In particular, a self-assembled monolayer of 1.5
.mu.m polystyrene microspheres on an aqueous surface can be
transferred onto the FTO substrate surface.
[0076] Next, as shown in FIG. 6B, the diameters of the polystyrene
beads are tailored by a process of inductively coupled plasma
reactive ion etching (ICP-RIE) with oxygen. In particular, the
diameters of the polystyrene beads can be tailored by ICP-RIE with
50 sccm of oxygen and 5 sccm of tetrafluoromethane at a pressure of
100 mTorr and radio frequency (RF) power of 100 W with 480 s of
etch time.
[0077] Next, as shown in FIG. 6C, a 150 nm layer of copper can be
deposited by an e-beam evaporation at room temperature. The 150 nm
Cu layer acts as a sacrificed mask.
[0078] Next, as shown in FIG. 6D, the monolayer of polystyrene
spheres 610 can be removed by ultrasonication in tetrahydrofuran.
The result is a plurality of holes 620 in the 150 nm copper layer
in the predetermined regions.
[0079] Next, as shown in FIG. 6E, the substrate 600 can be immersed
into a solution of 1% ML-371 at 45.degree. C. for 5 min.
Immediately after, as shown in FIG. 6F, the substrate can be
immersed into a solution of PVP-capped platinum nanoclusters
suspension (pH=2.3) at room temperature for 5 min and then annealed
at 250.degree. C. for 15 min. A nanoparticle dot-pattern can be
self-aligned on the FTO substrate surface wherein the platinum
nanoparticles are adhered by the ML-371 surfactant on the
predetermined regions of the substrate surface exposed by the
plurality of holes 620. The Cu layer can be simultaneously
dissolved in the acid PVP-Pt solution. The patterned seed layer of
nanoparticles can be then immobilized on the substrate by the
annealing process.
[0080] Finally, as shown in FIG. 6G, a desired material, typically
a metal, can be selectively electroplated onto the substrate 600 at
predetermined regions at an operating voltage within the optimum
range.
[0081] The results of the above deposition method can be verified
through scanning electron microscope (SEM) images. FIG. 7A-7E
depict the SEM images of a plurality of predetermined regions
selected for metallization. In particular, as shown in FIG. 7A,
holes 620 are form in a range of 500 to 700 nm. Shown in FIG. 7B, a
nanoparticle dot-patterns can be generated on the substrate 600
after the immersion into the nanoparticle solution and the
annealing treatment. As shown in FIGS. 7C and 7D, nickel can be
then electroplated at an operating voltage of -0.8 volt for 15
seconds and 60 seconds, respectively, to yield metal islands in the
predetermined regions. As shown in FIG. 7E, a dot array of
electroplated nickel having an average diameter of approximately
600 nm are formed and are spaced approximately 1 .mu.m apart. FIG.
7F depicts another confirmation of the selective metallization
process, as discussed above, through an AFM image and a
corresponding line scan of a post-deposition surface pattern, which
demonstrate that metal dots having a thickness of approximately 200
nm have been formed cleanly on the FTO substrate surface.
[0082] Furthermore, pull-off adhesion tests can be performed. For
example, a pull-off adhesion tests for electroplated nickel on FTO
glass with the nanoparticle bostontreatment showed that no
deposited nickel can be stripped off by a 3M Flatback Masking Tape
250 (ASTM D3359). In contrast, without the nanoparticle treatment,
almost all of the deposited nickel on the untreated FTO glass came
off.
[0083] According to a third embodiment, as shown in FIGS. 8A-8C,
there is provided a method for forming an electrode during the
fabrication of thermoelectric devices or device components. The
method described below deposits a seed layer which provides
nucleation sites for electroplating metal on nanostructured
Bi.sub.2Te.sub.3-based alloys in order to form good electrical
contacts.
[0084] A substrate can be prepared by any number of conventional
methods. For example, according to a preferred embodiment, bulk
nanostructured Bi.sub.2Te.sub.3-based materials, p-type (alloyed
with Sb as Bi.sub.xSb.sub.2-xTe.sub.3) and n-type (alloyed with Se
as Bi.sub.2Se.sub.yTe.sub.3-y) disk samples of 25 mm in diameter
and 2 mm in thickness are made by a ball milling and hot pressing
method. After hot pressing, both sides of the disk sample are
polished using sand paper. To achieve a wettable surface for
subsequent metallization, a 1 minute immersion in a 0.5%
bromine-ethanol solution at room temperature and a 5 minute
immersion in a 5% WAKO CLEAN-100 at 45.degree. C. with sonication
are used for surface cleaning.
[0085] First, as shown in FIG. 8A, a substrate 800 can be
pretreated by a deposition of a functionalized self-assembled
monolayer (SAM) 810 of 3-mercaptopropyl-trimethoxysilane (MPS). In
particular, the substrate 800 can be immersed into a 1% MPS
solution in ethanol for 50 minutes. The general chemical formula
for these silane-based self-assembled MPS can be R'--Si(OR).sub.3,
where R' is usually a short carbon chain containing some functional
groups and OR is a hydrolysable end-group such as ethoxy, methoxy,
or chloro group which can react with a hydroxylated surface on the
substrate to form silanol (Si--OH) groups.
[0086] The silanol groups react with the hydroxyl groups present on
the substrate surface to form interfacial covalent bonds.
Subsequent adsorptions result in the silanol groups condensing with
each other to form a polysiloxane (Si--O--Si) network.
[0087] Next, as shown in FIG. 8B, the pretreated substrate surface
can be immersed into a metal electrolyte for 30 seconds to form
bridging links between the monolayer 810 and the metal ions before
electroplating. In particular, the metal electrolyte can be a
nickel ion electrolyte.
[0088] Here, the functional groups protruding from highly ordered
and oriented SAMs produce a specific interaction function. Also, as
it was discovered, a SAM is able to transport electrons by a
hopping process. Therefore, according to the third embodiment, this
hopping process can be combined with the nucleation function of the
seed layer in the electroplating process by arranging a
hydrolysable end-group and bridging functional group on SAMs.
[0089] As shown in FIG. 8B, the SAM adheres to the semiconductor
substrate 800 and its functional group forms a bridging link to
metal ions (Ni.sup.+) in the electrolyte. According to the
preferred embodiment, MPS with methoxy end-groups and a sulfur
functional group can be used to form a SAM on nanostructured
Bi.sub.2Te.sub.3-based alloys. Furthermore, the silane-based MPS
can be characterized according to conventional spectroscopic
techniques such as the Auger electron spectroscopy (AES). For
examples, the reference Surface Characterization Using AES for
Silane provides a description of such technique. Additionally, a
further advantage of MPS can be that it forms a neutral solution in
alcohol, thereby avoiding oxidation and corrosion to the
substrate.
[0090] Next, as shown in FIG. 8C, a predetermined metal can be
electroplated onto the pretreated substrate surface within an
optimum range of the operating voltage. Here, nickel can be
electroplated. A second metal can be optionally electroplated to
achieve a desired electrical, chemical, or mechanical
characteristic for the contact (not shown). For example, gold can
be electroplated on the electroplated nickel to form a second
metallization layer.
[0091] Next, FIG. 9A depicts two cathodic waveforms obtained by
performing cyclic voltammetry on the process of electroplating
nickel on nanostructured Bi.sub.2Te.sub.3-based alloys with a
pretreatment as discussed above (waveform A), and for
electroplating nickel without the pretreatment (waveform B). Here,
a Gamry PCI4/300 potentistat/galvanostat can be used in the
electrochemical measurements in a standard three-electrode system
with a platinum mesh as the counter electrode and a saturated
calomel electrode (SCE) as the reference electrode.
[0092] As shown in FIG. 9A, the current-potential curve of
electroplating nickel on a pretreated substrate surface is more
positive (waveform A) than the current-potential curve of
electroplating nickel on an untreated substrate surface (waveform
B). Here, the pretreatment of the MPS monolayer provides nucleation
for electroplating metal, which can determined by the formation
energy, excess energy, and internal strain. Accordingly, the MPS
monolayer with a bridging sulfide group provides more nucleation
sites to increase the binding energy between metal nucleus clusters
and the substrate. This increase in binding energy decreases the
excess energy needed for electroplating nucleation, making the
potential more positive (UPD).
[0093] Therefore, nickel atoms more strongly adhere to a
nanostructured Bi.sub.2Te.sub.3 surface that is pretreated with a
MPS monolayer. In contrast, the nickel atoms would more strongly
adhere to each other than to an untreated nanostructured
Bi.sub.2Te.sub.3-based alloy. The rather high excess energy of
electroplating nucleation corresponds to an instantaneous
nucleation model, where the growth of nuclei on a small number of
active sites, such as crystal defects, atomic step or impurities,
occurs in a very short time period. These nucleation sites grow
into three-dimensional islands and then coalesce (Volmer-Weber
growth). Island growth during electroplating nucleation is usually
not desirable for technological applications due to its poor
adhesion and non-uniform deposition.
[0094] Pull-off adhesion tests using 3M Flatback Masking Tape 250
(ASTM D3359) also showed that no coating was stripped off for a 1
.mu.m layer of electroplated nickel on nanostructured
Bi.sub.2Te.sub.3-based alloys with the MPS treatment, but almost
all of the coating on the untreated nanostructured
Bi.sub.2Te.sub.3-based alloy came off.
[0095] As shown in FIG. 9B, a chronoamperograms can be obtained to
show that the current response to the optimum operating voltage at
0.85 volt for electroplating nickel on nanostructured
Bi.sub.2Te.sub.3-based alloys with a MPS monolayer treatment
(waveform A) and without a MPS monolayer treatment (waveform B). As
shown, the rate of electroplating can be 2.5 times faster in the
case of a MPS monolayer treatment.
[0096] Further as confirmation of the metal deposition method
described above, shown in FIG. 10A, an AFM image can be obtained
for a 1 .mu.m thick layer of electroplated nickel on an untreated
nanostructured Bi.sub.2Te.sub.3-based alloy. The AFM can be used to
scan the surface and to calculate a root mean square (RMS)
roughness, where a random distribution of only a few active sites
would cause irregularities in the grain size and increase film
roughness. Here, uneven grain growth also leads to stacking faults
thereby increases film stress and strain due to mismatch in the
lattice spacing. Therefore, the poor adhesion can be understood in
terms of weak interface contact and the large strains and stresses
in the electroplated film.
[0097] As shown in FIG. 10B, a RMS roughness can be reduced from
21.7 nm to 9.4 nm over a 25 .mu.m.sup.2 area when a MPS monolayer
is deposited as a seed layer for nickel electroplating. Here, 1
.mu.m electroplated nickel films on substrate surfaces with a MPS
monolayer treatment is compared to one that results without a MPS
monolayer treatment by SEM observations (FIGS. 10C and 10D,
respectively).
[0098] As shown in FIG. 10C, on an untreated substrate surface,
secondary nucleation atoms must travel towards each other on a
small number of nucleation sites in order to minimize surface
energy. Islands then grow to form a porous and irregular structure.
In contrast, a complete coverage by a dense and conformable nickel
film can be seen in FIG. 10D for a substrate surface that is
treated with the MPS monolayer.
[0099] Furthermore, according to a preferred embodiment, a method
of fabricating stable and low-resistance ohmic contacts is
provided. In order to form the desired contact, a low ohmic contact
at each end of the Bi.sub.2Te.sub.3-based alloys requires a
resistance of about 1.times.10-6 ohm-cm.sup.2, which represents
approximately 1 percent of the total resistance. Under this
condition, the impact of contact resistance can be safely neglected
and the ratio of ZT.sub.eff to ZT equals to unity.
[0100] Table 1 (below) shows data of electrical contact resistance
measured by a 4-wire AC method (Keithley 2300) for small cubes cut
from a disk of n-type or p-type nanostructured Bi.sub.2Te.sub.3,
both sides of which are treated by a MPS monolayer followed by
electroplating a 1 .mu.m layer of nickel and a 1 .mu.m layer of
gold. A small current (Joule heating can be neglected) with
alternating polarity under a high frequency of 1000 kHz (the
Peltier heat cancels due to periodic heating and cooling at the
junction) from 0.03 mA to 0.1 mA was input into the sample from the
top to the bottom. Total resistance can be obtained by the slope of
the voltage drop and the current. The intrinsic resistance of
nanostructured Bi.sub.2Te.sub.3 alloys can also be obtained by
measuring the voltage drop across the sample body itself using a
known distance. Therefore, the electrical contact resistance for
both sides is the difference between total resistance and intrinsic
resistance.
[0101] As shown in Table 1 below, the specific contact resistance
of electroplated nickel/MPS/nanostructured Bi.sub.2Te.sub.3 alloys
can be approximately 1.times.10-6 .OMEGA.cm.sup.2 and contributes
around 1% of the total resistance. Thus, the contact resistance can
be safely neglected and an ohmic contact is provided.
TABLE-US-00001 TABLE 1 Length Width Height BiTe Resistivity Total
Rs Specfic Contact Rs Contact/Total Type No. (mm) (mm) (mm) (Ohm
cm) (Ohm) (Ohm cm.sup.2) (%) N 1 1.07 1.06 1.54 1.29E-03 1.76E-02
8.79E-07 0.89 2 1.11 0.96 1.52 1.04E-03 1.48E-02 4.21E-07 0.54 3
1.21 1.21 1.48 1.08E-03 1.09E-02 2.92E-07 0.37 4 1.07 1.26 1.56
1.23E-03 1.44E-02 1.07E-06 1.11 P 1 0.89 1.01 1.48 1.17E-03
1.95E-02 1.02E-06 1.18 2 1.13 1.19 1.54 1.10E-03 1.27E-02 6.88E-07
0.81 3 1.20 1.20 1.69 1.00E-03 1.19E-02 9.90E-07 1.17 4 0.80 1.23
1.50 1.18E-03 1.82E-02 1.17E-06 1.32
[0102] Therefore, a method is provided for depositing an adhesive
and uniform metallic layer onto a nanostructured
Bi.sub.2Te.sub.3-based material. This method results in improved
electrical and thermal conductivity at lower cost and higher
throughput. Unlike sputtering, the deposition of MPS SAMs on a
substrate can be achieved using a wet process, which can be
significantly more cost effective. The thin MPS monolayer not only
provides a good adhesive layer but also sufficient nucleation sites
for electroplating, thereby improving adhesion and uniformity.
Measurements of contact resistance and the device efficiency for a
solar thermoelectric power generator are also confirmed to yield
results of similar quality compared with those created conventional
methods.
[0103] The new method has many distinct advantages over existing
processes during the metallization step and would be applicable in
the manufacturing of various substrates. One utility of this method
can be the fabrication of metallic electrodes onto other
thermoelectric materials, such as lead telluride, zinc telluride,
and so on. Other examples include electroplating metals on
conductive glass and other low-roughness ceramics for solar panels,
light emitting diode (LED) wafers, and the solder bumping process.
Additionally, some SAMs, such as octadecyl trichlorosilane (OTS),
can be used for nanolithography by localized probe oxidation via
AFM, STM or electrochemical methods. By integrating
bridging-function SAMs and nanolithography SAMs, other potential
applications include the creation of patterned electrically driven
sensing surfaces, glass circuit boards for high density electronics
packaging, microscale resistive heaters, transparent
micro/nanoelectrodes, and surfaces where spatially patterned work
functions could serve as templates for subsequent patterning.
* * * * *